1. Technical Field
The present invention relates in general to an assembly fixture, and in particular to an improved fixture for assembling pivot bearing assemblies for disk drives. Still more particularly, the present invention relates to a self-aligning fixture for pre-loading and aligning pivot bearing assemblies for disk drives.
2. Description of the Prior Art
Referring to FIG. 1, a schematic drawing of an information storage system comprising a magnetic hard disk file or drive 11 for a computer system is shown. Drive 11 has a base 13 containing magnetic disks 15 that are rotated about a hub 17. A set of actuator arms 21 are pivotally mounted to base 13 on a pivot assembly 23. A controller 19 is mounted to base 13 for selectively moving arms 21 relative to disks 15. Each arm 21 comprises a mounting support 25, a suspension 27, and a head gimbal assembly 29 having a magnetic read/write head secured to each suspension 27 for reading data from and writing data to disks 15. A conventional voice coil motor 26 is also mounted to pivot assembly 23 opposite head gimbal assemblies 29. Movement of an actuator driver 28 (indicated by arrow 30) moves head gimbal assemblies 29 across the disks 15 until the heads settle on the target tracks.
As shown in FIG. 2, pivot assembly 23 contains ball bearings 31, a shaft 33, and a sleeve 35. The naturally-occurring, micron scale, machine tolerance inaccuracies in these components and in the tolerances of a conventional fixture 37 used to pre-load pivot 23 (shown greatly exaggerated for clarity) accumulate during the assembly of pivot 23. The lower end of shaft 33 mounts in a hole 41 in base 43 of fixture 37. Sleeve 35 is suspended above the upper surface of base 43 as a tubular anvil or rod 45 exerts axial force on the upper face 46 of the inner ring 47 to pre-load pivot 23. Bearing pre-load is controlled to maintain sufficient stiffness of the pivot assembly. Fixture 37 only has one degree of freedom (up and down) to apply the pre-load to face 46 of inner ring 47.
The tolerance stack-up of pivot 23 and fixture 37 causes significant angular misalignment between inner and outer rings 47, 49. This misalignment or xe2x80x9cring face out-of-parallelismxe2x80x9d creates a moment on one side of inner ring 47, thereby producing a greater localized pre-load force on the ball compliment. The term xe2x80x9cball complimentxe2x80x9d is used to describe the array of spaced-apart balls within a bearing. Unfortunately, because fixture 37 is non-compliant, it is unable to compensate for ring face out-of-parallelism, raceway wobble and groove wobble. When a non-uniform, pre-load force is exerted on the ball compliment, one side is pinched tight by the raceways while the opposite side is loose. As pivot 23 rotates, the balls are compressed as they pass through the tight zone, thereby causing torque ripple. This phenomena is an excellent indicator that the actuator itself is dynamically unstable. Torque ripple must be compensated by the servo controller by varying the coil current. This non-uniform, pre-load force around the circumference of the bearing corresponds to a non-uniform stiffness around the circumference. Stiffness variation from pivot to pivot or circumferentially within one pivot can affect the frequencies and gains of the structural resonances of the actuator.
The problem of angular misalignment of the raceways can be prevented by increasing the radial clearance or gap between the shaft and the inner diameter of the bearing. A larger gap allows more compliance between the parts. Unfortunately, this solution creates the problem of increased outgassing arising from the additional adhesive required to fill the larger gap between the bearing and the shaft. Thus, an improved solution for pre-loading pivot assemblies is needed.
A fixture for pre-loading a pivot assembly has a stationary base and a tool that is axially movable relative to the base. The tool has a platen with a pre-load mass and a cylindrical enclosure. A sphere is suspended within the enclosure but is free to roll and move laterally along three degrees of freedom. The pivot assembly has an external sleeve with a coaxial shaft mounted on bearings. Each ball bearing has an inner ring, an outer ring, and balls in between. The rings are adhesively bonded to the shaft and sleeve.
The pivot is placed on the base of the fixture and the tool is lowered. The sphere is free to xe2x80x9cfloatxe2x80x9d within the enclosure until it makes contact with the inner ring of the upper bearing in the pivot. Upon contact, the compliant, rolling sphere automatically self-aligns with the pivot. The pre-load mass applies the desired force against the pivot until the adhesive cures. The sphere ensures that the pre-load force is uniform around the ball compliment circumference of the bearings such that the inner and outer rings of the bearings float and their raceways self-align under the pre-load force. After the adhesive cures, the pivots have negligible torque ripple and consistent dynamic response.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings.